In recent years, the RFID (Radio Frequency IDentification) has been used in various fields. The RFID is a technique to read out data including unique identification information stored in an IC (Integrated Circuit) tag by radio communication and to write data into the IC tag.
The RFID includes the active type in which the power for operation is supplied internally, and the passive type that has no battery inside and operates with the power supplied by the high frequency wave transmitted from a reader apparatus.
For the passive type that has no battery, the use in various fields including the logistics field has been expected, since it may be offered at a relatively low price as compared to the active type.
In addition, when the UHF band (860-960 MHz) is used as the frequency band for the RFID, even with the passive type, the reading area is relatively broader compared with other frequency bands, and a plurality of tags may be read at one time.
Therefore, in the logistics field for example, it becomes possible to read a plurality of tags attached to a number of items at one time, to inspect the items.
When the reading area is broadened using the UHF band, information of a tag that is not intended by the administrator may be read. For example, in inspecting arriving items and shipped items in a warehouse and the like, for a tag of an item placed on a position that is usually too far from the inspection gate to read, with a forklift and the like passing nearby the item, the information of the tag may be read due to radio wave reflected on the forklift. In addition, when a plurality of gates are placed, there is a possibility that an erroneous judgment is made, as the tag of an item entering an adjacent gate may be read.
In such a case, the unwanted tag may be excluded by performing filtering based on the ID stored in the tag. As an example, in a case where the IDs are hierarchized by category of the item (for example, the type representing whether it is a pallet or an individual item), by knowing category data representing the category of the item in advance, the tag ID indicating a pellet tag may be excluded.
In addition, when the unintended tag reading is due to the reflection of radio wave, a method has been known in which a plurality of detections are performed regularly for the tag attached to an item, and when the ID of the tag could not be detected successively for predetermined times, it is excluded as read accidentally due to reflection (for example, see Japanese Laid-open Patent Publication No.: 2005-275960).
However, when the target tag and the unwanted tag is an item of the same category, the sorting by category data included in the hierarchized ID is impossible.
Other than that, in order to exclude the unintended tag reading, there is a method in which the reading area is isolated physically by a radio wave absorption plate and the like, this has a problem that the man hour for the work at the installation site increases significantly.
Furthermore, there is a method in which a tag outside the original reading area is found and regarded as an unwanted tag by using an antenna such as the phased array type with which its directivity may be changed, but this newly causes a problem that the reading apparatus becomes expensive.
In order to solve these problems, the inventors of the present application have disclosed, in the prior applications, a method in a reader (reading apparatus) that repeatedly read data in a contactless manner from a tag in a communicable area and a control system, the result of clustering by degree of similarity given to time series data from time series data of reading results collected in advance is held as reference data, and sorting into necessary data or unwanted data is performed by calculating the degree of similarity of read data and the reference data (see Japanese Patent Application No. 2008-298770 and Japanese Patent Application No. 2009-999924).